Myocardial ischemia/reperfusion (IR) injury is the main clinical challenge of adverse cardiovascular outcomes after myocardial ischemia, cardiac surgery or circulatory arrest. It is well established that myocardial ATP depletion is a key feature of myocardial ischemia and heart failure1. Improving myocardial energy metabolism has been a well- known target to protect the heart from IR injury, but with little success. ATP synthase is a key enzyme complex generating ATP in mitochondria, thus playing a central role in mitochondrial function. Functional defects of ATP synthase can cause and aggravate human diseases, such as cardiomyopathy and congestive heart failure. However, our understanding of the regulation of energy metabolism and mitochondrial function in the energy demanding heart remains poor. We have recently identified a PPAR-target gene encoding a novel mitochondrial protein ES1 with unknown function. Preliminary studies revealed that ES1 is a mitochondrial protein interacting with the subunits ? and ? of ATP synthase F1 sector. We were excited to find that ES1 works as an enhancer of ATP production by increasing ATP synthesis and inhibiting ATP hydrolysis. However, it remains unknown if ES1 similarly regulates ATP production in the heart subjected to myocardial IR. Interestingly, our preliminary studies revealed that ES1 protein levels were decreased in hearts with myocardial IR injury of patients and mice. Based on the pilot studies on conditional transgenic and gene targeting mouse lines, we hypothesize that ES1 is a novel therapeutic target of protecting the heart from myocardial IR via its role in facilitating cardiac energy production/reservation. To test this central hypothesis, we will first define the role of ES1 as an endogenous regulator of ATP synthase and as a determinant of mitochondrial structure/function in the heart. We will then determine if transgenic and adenoviral-associated virus-mediated ES1 overexpression in the heart protects the heart against myocardial IR injury by its role in regulating energy metabolism. These studies will provide novel insights into how to manipulate energy metabolism to protect the heart from myocardial IR injury. Furthermore, these new fundamental scientific findings will have broader implications in diseases related to other organs and tissues.